In medical imaging the diagnosis is usually made by radiologists. When the images are available in computer readable form (digitized), the analysis of the images may be performed while they are displayed on a video monitor. Scientists in the medical diagnostic imaging field are continuously working to improve the quality of the images, acquired by the various modalities in the field. One of the well-known methods for improving the detectability of lesions of a displayed organ is by “windowing”.
Windowing describes the process of displaying the detected data by extending certain detected data ranges to fill the entire intensity range of the displayed image. For example, there are only a limited number of gray levels which may be distinguished, this being due to inherent limitations of the CRT and the human eye. Therefore, it is very important to utilize the available gray levels in the regions of data values which carry the most useful information. To illustrate, assume that a black and white system has the capability of displaying 256 gray levels. If the acquisition of data occurring during an examination of a subject results in only 86 gray levels being used that are clustered away from the lowest and the highest levels; then, only about one-third of the system's contrast display capability would be used.
Windowing is presently accomplished by using a table-look-up or look-up table(s). When displaying an image, the data is stored in a digital memory, and is read out repeatedly to refresh the video display. For that purpose special circuits are used to control the read out sequence, to synchronize the raster monitor timing, to convert the digital data into analog data, etc. These circuits are well known to those skilled in the art.
To perform the “windowing” function using table look up techniques, a translation table is stored in a memory within the electronic circuitry responsible for the display refresh. The image data, on a pixel-by-pixel basis is used as an address and applied to the memory. The corresponding gray level stored in the memory is readout and used to control the intensity of the displayed pixel. The LUT usually has two parameters: a slope and a threshold that are determined by an (human) operator. Those parameters can be adjusted using either knobs or a software user interface until the image enhancement is satisfactory.
The problem with windowing is that it introduces visual artefacts like e.g. the distribution of shades is not maintained throughout the image for which the contrast is enhanced.
There is still a need for improvements.